nimh cell matching

[Fishstalker]

Hi guys im looking for a bit of advice on making a nimh battery pack.
Im planning on buying some el cheapo c cell nimh batteries off ebay and making it into a battery pack, 24V 9AH 20 cells in series and ive been doing some research.
I keep hearing about this cell matching and cell balancing thing, im just wondering how to do it and will it eat up alot of time? Because im trying to set up a little electric folding bike for my gf and id like to make things as simple for her as possible.
So i thought id make that battery pack from nimh and get one of those 24V nimh smart chargers but then i saw that cell balancing and matching thing and how neglecting to do it shorten the life of the batteries. (i only have a general idea of what those things mean)
Any thoughts or suggestions guys? Thanks

(ps if anyone can tell me where to get one of those aforementioned smart chargers in australia id greatly appreciate it, all ive been able to find are ones from overseas =( )

 

[Ykick]

Sorry can't help you with sourcing chargers but I've worked for many years/miles with 40S NiMh and I can tell you that C cells sound kinda small to me. I'm accustomed to F cells and they're only rated 1C or about 13A max. I've pulled 2-3C (25-35A) from them for years and they're still running pretty strong but I'm doubtful C cells are going to match that performance?

Balancing is easier than the lipo stuff because NiMh will allow shuttle charges to pass but you need temperature as well as Delta V drop circuit to terminate balance charging. I often bulk charge all 40 cells in series using timed cutoff with an old SLA charger but for balancing I only charge 10S with the NiMh charger(s) I got from BatterySpace.

NiMh is fairly forgiving in that you can drain them completely flat and they seem to handle mild overcharging. Downside is that they lose 10% of their charge sitting for a day until they reach about 50-70% SOC. After which point they discharge at a much slower rate but will drain completely empty if sitting long enough. They're not good leaving sit for days before use.

Regular daily use is the best way to use NiMh. Very durable battery chemistry as mine are about 4 years old and still deliver 7-8AH from 13Ah F cells and I've abused them in more ways than I care to admit.

good luck

 

[amberwolf]

I don't recommend the ebay NiMH; you're unlikely to get anywhere near the rated capacity even just using them in flashlights, and I doubt you'll see even 1/10 of it as an ebike pack.

I use a 36V 9Ah D-cell pack, and have a 24V 13Ah F-cell pack that will see some use soon enough (both well used before I got them, but still working well).

I also have a few wheelchair sub-C 24V packs, rated about 2.2Ah, but not yet tried to use them for anything. I expect as long as they're not drained quickly they might actually get something like their rated power, because they use cells meant for the purpose.

Since you can't easily just parallel NiMH cells, then to get sufficient power for an ebike to be useful with the ebay NiMH you'd need to series a lot of cells, for a very high voltage, to get useful Wh. Then you need a controller rated for that high voltage (though one that can be set for very low current limit, on the order of <2 to 5A), and a motor wound to run at that voltage when spinning at the max speed you want the bike at. And expect the cells to drop from around 1.4-1.5V each while on the charger, to 1V during use, as most of the cheap ones don't appear to be rated for even 1C use based on what I have read of people's results with them so far.

*Good* NiMH cells can make a good pack. I doubt the cheap ones will.